Gray irons suitable for automotive castings in general, and hardenable alloy gray irons used in making heavy duty automotive camshafts in particular, have heretofore been produced by melting a charge of iron and steel materials in a cupola. During such processing, it has been found necessary to add costly alloys, in closely controlled amounts, to the cupola to produce a machinable casting having the desirable wear and strength properties.
Two known chemical compositions for hardenable alloy gray irons in percent by weight are set forth below:
______________________________________ Gray Iron A Gray Iron B ______________________________________ Total Carbon 3.10 - 3.45 3.10 - 3.40 Silicon 2.10 - 2.40 1.70 - 2.00 Chromium 0.85 - 1.20 0.40 - 0.60 Manganese 0.60 - 0.90 0.55 - 0.85 Molybdenum 0.40 - 0.60 0.42 - 0.58 Nickel 0.20 - 0.45 1.00 Min. Phosphorus Trace - 0.20 Unknown Sulphur Trace - 0.20 Unknown Iron Remainder Remainder ______________________________________
Unfortunately, even though the chemical composition of the cupola charge is closely controlled to provide the above-mentioned percentages, the castings which are produced continue to vary greatly in the characteristics of the microstructure and particularly do not meet the relatively high controlled carbide content desired. In some cases the chromium and molybdenum percentage levels have been balanced by costly nickel additions to produce the required mechanical properties of the casting such as high tensile strength, with the promotion of relatively significant percentages of controlled carbides along with the accompanying increased wear resistance not being a factor. In other cases the castings which have been produced have frequently had a secondary carbide or controlled carbide content below the desired percentage range, e.g., 20 percent to 40 percent, with the result that such castings have been scrapped because they were too soft and would experience a high rate of wear. Such higher rates of scrap, and the use of expensive alloy additions in an attempt to obtain the relatively high controlled carbide content, have raised the cost of these castings and this has forced the industry to accept castings of poorer quality than that desired.